Heat radiation tubes are mainly used in furnaces where the furnace atmosphere does not allow direct heat. This can be due to the atmosphere being harmful to the elements which are being used for electrical heating or due to the desire to control the atmosphere in the furnace whereby combustion gases are not allowed therein. Other reasons for the use of radiation tubes instead of direct heating where such should be possible might be for example that one wants to repair or exchange the heat source while the furnace is being used. It will then be easier to do this in a separate space, e.g. inside the radiation tube, than in the furnace chamber itself.
A heat radiation tube may comprise a cylindrical tube. A bottom or end plate is mounted in one end of the tube. In the other end of the tube there is, as a rule, a flange for mounting to the furnace wall. The tube can also have other arrangements, protrusions, etc. for mounting in the furnace as well as distance pieces and the like. Mainly when heating is obtained by combustion there may be inserts in the tube forming flow channels for the combustion gases. U-shaped radiation tubes may also be used.
Radiation tubes have hitherto mainly been used at furnace temperatures up to about 1100.degree. C. The known tubes are often made from an alloy mainly comprising nickel, chromium and iron. The alloy composition is for example 40-60 weight % nickel, 15-20% chrome and 25-45% iron. These radiation tubes, however, have certain drawbacks which are of great importance in most applications. On the outside as well as the inside surface of the tubes oxide layers are formed which are spalled off when they have reached a certain thickness, which varies due to conditions in each application. In these tubes the oxide layers fail to provide a protective layer. Falling oxide flakes may cause problems if they get into contact with the products present inside the furnace. However, the greatest problems are caused by the oxide flakes on the inside surface of the tubes. If these surfaces are holding electrical heating elements, the oxide flakes may cause short-circuiting between separate elements and between separate parts of one element which brings with it an immediate interruption of the function of the element or a considerably decreased useful life of the element. When an element is exchanged, which means that element and element support is pulled out from the radiation tube and after repair or exchange is again reinserted into it, the supports may function as scrapers and cause large amounts of oxides to accumulate in most cases in the distant end of the tube which may cause difficulties during the repair work and cause malfunctions.
Hitherto used radiation tubes do not have satisfactory mechanical properties at high temperatures. Due to their own weight and the internal load the tubes tend to sag. In order to compensate for this sagging the tubes have to be turned 180.degree. at regular intervals. This can, in most cases, be made in connection with normal maintenance or repair but it is still an essential drawback and a factor which limits the possibilities of use.